In recent years, the technology for causing networked devices to autonomously cooperate with each other has been considered with an aim to improve intelligence, functionality, user convenience or energy efficiency. In this technology, it is preferable that each device communicate autonomously as much as possible to save the user's time.
Meanwhile, in order to improve energy efficiency, each device preferably reduces the amount of communication as much as possible. In addition, when a user is not using a device, it is preferable that the device be in low operating mode in which message transmitting and receiving is not performed at all. Such low power operating mode is commonly known as sleep mode.
Devices having sleep mode have two modes: sleep mode during which messages are not, sent or received and active mode during which messages are sent and received. The devices wake up at a particular timing during sleep mode and switch to active mode. Furthermore, the devices switch to sleep mode at a particular timing during active mode.
Note that communications devices having two modes: sleep mode and active mode are described in the Patent Literature 1.